Problem:
make[1]: Entering directory `/home/nbdev/Downloads/glimpse-4.18.6/dynfilters'
flex -F -8 htuml2txt.lex
gcc -O3 -fomit-frame-pointer  -c lex.yy.c
gcc  -o htuml2txt lex.yy.o -lfl
/usr/bin/ld: cannot find -lfl
collect2: error: ld returned 1 exit status
make[1]: *** [htuml2txt] Error 1
make[1]: Leaving directory `/home/nbdev/Downloads/glimpse-4.18.6/dynfilters'
make: *** [build-sub] Error 2

Building glimpse-4.18.6 on CENTOS 7 fails looking for flex lib (-lfl)
Flex is installed.

Solution:
yum install flex-static
this will put the libraries in the expected place